Lipidosis, characterized by the accumulation of lipids such as phospholipids, neutral fats, and sphingomyelin in living tissue with the administration of a drug, is called phospholipidosis, steatosis, sphingolipidosis and the like, respectively, according to the kind of accumulating lipid, and is also generically referred to as drug-induced lipidosis. Many of lipidosis-inducing compounds have a structure wherein both a hydrophobic region and a positively charged hydrophilic region in the molecule, i.e., what is called a cationic amphiphilic drug (CAD) structure.
In recent years, with the advance in genome analysis, orphan receptors have been recognized as valuable as drug innovation targets, and receptor agonists or antagonists have been developed, but some of such compounds have a CAD structure, which sometimes leads to the induction of lipidosis and eventually interferes with pharmaceutical development. Additionally, some of approved pharmaceuticals have been reported to cause lipidosis as an adverse drug reaction.
Currently, in a toxicity evaluation study of a pharmaceutical candidate compound, it is common practice to administer the compound to a laboratory animal such as the rat and examine histopathological changes by electron microscopy, but histopathological changes of lipidosis are often not manifested unless the compound is administered for a long time; furthermore, there are additional drawbacks, including the time and labor required for tissue specimen preparation and toxicity detection. In particular, to quickly predict the presence or absence of toxicity, and to efficiently optimize the structure, in the initial stage of drug innovation, it is essential to construct a screening system enabling the evaluation of multiple specimens more conveniently and in shorter time.
In diagnosing adverse drug reactions in toxicity studies in clinical studies and patients on medication, the applicability of the above-described method, which necessitates the collection of biopsy specimen, is largely limited because of the major surgical invasion. Therefore, there is an urgent demand for the development of an evaluation system enabling the efficient and non-invasive prediction or diagnosis of drug-induced lipidosis.
As an example of the non-invasive method of predicting and diagnosing lipidosis, a method comprising detecting the occurrence of cytoplasm-vacuolated lymphocytes in peripheral blood can be mentioned, but this is not only problematic in terms of efficiency as it necessitates microscopic examination of blood smear specimen and for other reasons, but also inadequate in terms of the reliability of prediction and diagnosis as it is known that a considerable percentage of lipidosis-inducing compounds do not cause the occurrence of vacuolated lymphocytes in peripheral blood and have only a particular organ as the target.
Metabonomics, which is to comprehensively analyze intermediate and final metabolites in peripheral humoral fluids (urine, plasma and the like), organs or cells, is coming to be utilized as an approach to monitoring changes in biological reactions, which will follow transcriptomics and proteomics, in various areas of medicine and biology. In the area of toxicology as well, this technology has begun to be utilized for research into the elucidation of toxicity development mechanisms and the prediction of toxicity; the technology, along with toxicogenomics and toxicoproteomics, is expected to find applications in drug safety evaluations and clinical diagnosis as a technology providing suggestion for molecular toxicological endpoints that will replace conventional toxicological endpoints (symptoms, laboratory testing, histopathological examination and the like) (see, for example, J. K. Nicholson et al., Nat. Rev. Drug Discov., 1: 153-161, 2002 and J. C. Lindon et al., Toxicol. Appl. Pharmacol., 187: 137-146, 2003).
Toxic phenomena are considered to be accompanied not only by independent changes in a single metabolite, but also by integral changes in various intermediate and final metabolites localized in a plurality of metabolic pathways. Hence, it is expected that using a technique enabling the simultaneous detection of signals from nearly all metabolites, such as nuclear magnetic resonance (NMR), will make it possible to comprehensively interpret the behavior of biological molecules involved in toxicity development.
Regarding association with drug-induced lipidosis, it has been reported that as a result of an analysis of urine from rats receiving drugs known to induce phospholipidosis (hereinafter also abbreviated as “PLsis”) using proton NMR (1H NMR), changes in various metabolites were observed, with phenaceturic acid (phenylacetylglycine; PAG) increased in common by drug administration (J. R. Espina et al., Magn. Reson. Chem., 39: 559-565, 2001), suggesting that PAG may be used as a biomarker for the potential of drugs for inducing PLsis. However, there is no knowledge about multiple drugs, and the reliability thereof as actual markers remains unknown (for example, in Society of Toxicology 43rd Annual Meeting Abstracts/Toxicology 194 (2004) 206-207, urinary PAG is described as being effective as a biomarker not only for PLsis but also for other lipidosis, whereas in Analytical Chemistry, 75: 4784-4792 (2003), PAG is judged to be no more than a weak biomarker for PLsis).